(a) Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof. More specifically, the present invention relates to a field sequential driving type liquid crystal display (FS-LCD), and a driving method thereof.
(b) Description of the Related Art
As personal computers and televisions, etc. become more lightweight and thin, their display devices should also be lightweight and thin. Accordingly, flat panel displays such as liquid crystal displays (LCDs), which are both light weight and thin, have recently been developed for these computers and televisions in place of cathode ray tubes (CRTs).
An LCD is a display device that obtains a desired video signal by applying and controlling the strength of electric fields to liquid crystal materials of anisotropic dielectric constants injected between two substrates so as to control an amount of light transmitted through the substrates from an external light source (e.g., backlight).
The LCD is representative of an example of portable flat panel displays, and, among LCDs, a TFT-LCD using thin film transistors (TFTs) as switching elements is mainly used.
Each pixel in a TFT-LCD can be modeled with capacitors having liquid crystal as a dielectric substance, such as a liquid crystal capacitor. An equivalent circuit of each pixel in such an LCD is shown in FIG. 1.
As shown in FIG. 1, each pixel of the liquid crystal display includes a TFT 10 having a source electrode and a gate electrode that are respectively connected to a data line (Dm) and a scanning line (Sn). The pixel also includes a liquid crystal capacitor Cl connected from the drain electrode of the TFT 10 to a common electrode where voltage Vcom is applied and a storage capacitor Cst connected to the drain electrode of the TFT 10.
In FIG. 1, when a scanning signal is applied to a scanning line (Sn) and the TFT 10 is turned on, data voltages (Vd) supplied to a data line are applied to each pixel electrode (not shown) through the TFT 10. Then, an electric field corresponding to a difference between pixel voltage Vp applied to the pixel electrodes and common voltage Vcom is applied to a liquid crystal (which is equivalently shown as a liquid crystal capacitor in FIG. 1). Light transmits with transmittivity corresponding to the strength of the electric field. In the pixel of FIG. 1, a pixel voltage Vp needs to be maintained during one (1) frame or one (1) field, and a storage capacitor Cst is used to maintain the pixel voltage Vp applied to the pixel electrode.
Generally to display color images, an LCD can use a color filter method or a field sequential driving method.
A color filter method forms color filter layers composed of three primary colors of red (R), green (G), and blue (B) in one of two substrates of the LCD, and displays a desired color by controlling an amount of light transmitting from the color filter layers. Specifically, to display a desired color, the color filter method respectively controls the amount of light transmitting from each of the R, G, and B color filter layers with light from a single light source transmitted through the R, G, and B color filter layers.
An LCD device displaying color using a single light source and three (3) color filter layers needs a unit pixel corresponding to each of R, G, and B subpixels, thus at least three (3) times the number of pixels are needed compared with displaying only black and white, therefore, making the techniques of manufacturing an LCD using the color filter method more complicated.
Further complicating the matters, there are problems in that separate color filter layers need to be formed on the substrate for an LCD during manufacturing, and the light transmission rate of color filter itself needs to be improved.
In contrast with a color filter method, a field sequential driving method sequentially and periodically turns on each independent light source of R, G, and B colors, and adds color signals corresponding to each pixel that are synchronized based on the periodic lighting time of the light sources to obtain full colors. That is, in a field sequential driving type LCD, one pixel is not divided into subpixels. The R, G, and B primary color lights outputted from R, G, and B backlights of the LCD are sequentially displayed in a time-division manner so that the color images are displayed by means of an after-image effect of the eye.
The field sequential driving method can be further classified into an analog driving method and a digital driving method.
In the context of the following discussion of the present application, driving voltage refers to a voltage applied to the liquid crystal, and optical transmittivity refers to a transmittivity of light when light is transmitted through the liquid crystal. That is, optical transmittivity is a torsion degree that allows a liquid crystal to transmit light.
The analog driving method establishes a plurality of gray voltages. The method then selects one gray voltage corresponding to gray data among the gray voltages, and drives a liquid crystal panel with the selected gray voltage to perform gray display with an amount of transmission corresponding to the gray voltage applied.
FIG. 2 shows a driving voltage and an amount of light transmission of a conventional liquid crystal display using an analog driving method.
Referring to FIG. 2, a driving voltage of V11 is applied to a liquid crystal, and light corresponding to the driving voltage of V11 transmits through the liquid crystal in an R field period Tr for displaying an R color. A driving voltage of a V12 level is applied to a liquid crystal, and light corresponding to the driving voltage of V12 level transmits through the liquid crystal in a G field period Tg for displaying a G color. And a V13 level driving voltage is applied to a liquid crystal, and an amount of light transmission corresponding to the V13 level is obtained. A desired color image is, thus, displayed by a combination of R, G, and B light transmitted respectively in Tr, Tg, and Tb periods.
In contrast with an analog method, a digital driving method applies a constant driving voltage to a liquid crystal, and controls a voltage applying time to perform a gray display. The digital driving method maintains the driving voltage, and controls the voltage applying state and voltage non-applying state with respect to their timing, so as to control an accumulated amount of light transmitting to the liquid crystal.
FIG. 3 shows a waveform for explaining a driving method of a liquid crystal display of a conventional digital driving method, and shows a waveform of a driving voltage and optical transmittivity of a liquid crystal based on driving data of a predetermined bit.
Referring to FIG. 3, gray waveform data corresponding to each gray is provided with a digital signal having a predetermined number of bits, for example, a seven (7) bit digital signal, and a gray waveform according to seven (7) bit data is applied to a liquid crystal. Optical transmittivity of a liquid crystal is determined based on a gray waveform applied to perform gray display.
Regardless of whether the driving method is analog or digital, in a conventional field sequential driving method, correct gray display is not possible since an effective value response of a desired gray for display (for example, a gray of R) is changed by a previous gray display (for example, a gray of G). That is, conventionally, a pixel voltage Vp actually applied to a liquid crystal is determined by both a gray voltage (or gray waveform) supplied to the present field (for example, an R field) and a gray voltage (or gray waveform) supplied to the previous field (for example, a B field).
U.S. Pat. No. 6,567,063 discloses a field sequential driving method using a reset pulse to solve the problem of the field sequential driving method in which an effective value is changed because of a previous gray display.
FIG. 4 shows a field sequential driving method using a reset pulse described in the U.S. patent. In FIG. 4, period (T31˜T36) indicates the R field, G field, and B field performing gray display for each R, G, and B. A predetermined voltage (reset voltage) is applied, which is independent of the input gray data, and is more than a maximum value of gray data applied during a predetermined time (T31˜T36) at the point where each of the periods (T31˜T36) are ended. A state of all liquid crystals is reset to the same state (e.g., a black state in which no light can be transmitted, that is, optical transmittivity is 0) at the point where each of the periods (T31˜T36) are ended.
Thus, when liquid crystals are driven by voltages applied with gray data at each period (T31˜T36), the liquid crystal state becomes the same state regardless of previous gray displays, and the display period for the present gray is not affected by the previous gray display. However, by turning off the backlights (RLED, GLED, BLED) so as to not allow light transmission during the period (reset period) when the reset voltage is applied, the period when each of R, G, and B light sources is turned on during each of the R, G, and B fields is shortened. Thus, during the reset period of the above method, there is a problem in which brightness is poor, compared with driving methods which do not use application of a reset voltage.